Sound Cancellation Techology

[Mark Moore]

Earmark, maker of audio trainers for educational use, makes a hearing 
protector incorporating noise cancellation technology.  The unit also 
incorporates a transmitter/receiver for two way communication.  This 
device is normally used in a high noise industrial setting where 
communication is vital.  This device is fairly expensive.
I also know of a company who makes a device for sound field use that uses 
cancellation technology.  This device is also used in industrial and 
medical settings.  
The technology incorporated in both of these devices is most effective in 
cancelling steady, repeating noise at low frequiencies like air 
conditioning noise, pumps, engines.  I am not familiar with high 
frequency applications.
I have heard people complain about disorientation while wearing noise 
cancelling headphones when moving.  I suppose that since the cancellation 
signal is generated from a microphone on the headset, and this signal 
changes in relation to the head, and there is a time lag between movement 
and compensation, this combines to create some confusion.  I'll leave 
that to a psychacoustician to explain.

Mark